Field of the Invention
The present invention relates to an imaging apparatus and a control method thereof.
Description of the Related Art
Imaging apparatuses for performing shooting using a solid-state imaging element such as a charge-coupled device (CCD) sensor, a complementary metal-oxide-semiconductor (CMOS) image sensor, or the like are being proposed. In the imaging apparatuses, various noises that cause image quality deterioration occur in a process in which an imaging element converts an optical image into an electric signal. Representative noise is reset noise of a pixel and a read circuit and random noise which changes every time an imaging operation is performed such as a dark current occurring in a pixel region. In addition, the imaging apparatus has a large number of elements which may serve as noise sources that periodically change a power supply voltage of the imaging element inside the imaging apparatus or in the vicinity of the imaging apparatus. When the noise sources operate during signal reading of the imaging element, the power supply voltage supplied to the imaging element changes due to a change in a power supply or electromagnetic waves. Because the noise sources are driven at a specific frequency for each device and each component, the power supply voltage of the imaging element periodically changes and the noise appears as noise of a periodic horizontal stripe shape in an image.
In Japanese Patent Laid-Open No. 2010-50636, an imaging apparatus for reducing horizontal stripe noise by changing a frequency of a noise source and a driving timing at which the imaging element is driven with respect to horizontal stripe noise due to the noise source which periodically changes is disclosed. In addition, in Japanese Patent Laid-Open No. 2001-124984, an imaging apparatus including an imaging element having two photodiodes (PDs) in one pixel is disclosed.
It is possible to obtain an A image signal obtained from the first PD and an A+B image signal obtained from the first and second PDs using an imaging apparatus of Japanese Patent Laid-Open No. 2001-124984. The A+B image signal can be used in a captured image signal. In addition, it is possible to obtain a B image signal according to a difference between the A+B image signal and the A image signal and detect a focus state based on a phase difference between the obtained B image signal and the A image signal.
In the imaging apparatus disclosed in Japanese Patent Laid-Open No. 2001-124984, it is necessary to perform reading of the captured image signal and reading of the focus state detection signal in time series. At this time, when the noise source for periodically changing the power supply voltage of the imaging element operates, horizontal stripe noise also occurs in the focus state detection signal in addition to the captured image signal. Accordingly, when the horizontal stripe noise occurs in the focus state detection signal, the accuracy of focus state detection is degraded. Because signal reading of the imaging element in which two PDs are provided in the pixel is not considered in the imaging apparatus disclosed in Japanese Patent Laid-Open No. 2010-50636, it is difficult to reduce horizontal stripe noise for both the captured image signal and the focus state detection signal.